Named Briney Eccentric Bore tooling, a new end mill holder design is said to reduce the TIR that is typical of end mill holders using a set screw or Weldon flat set screw by as much as 75 percent. According to the company, the out-of-concentric condition of the toolholder bore is improved from 0.0008" to 0.0002", and the holders feature an eccentric bore, imparting 0.0003" eccentricity in the direction of the set screw. The company-developed internal grinding process was designed to impart the feature consistently to any of Briney’s end mill holders, regardless of size, without affecting other toolholder features and tolerances.
By incorporating the eccentric bore, the clearances between the toolholder bore and the cutting tool can reduce the runout condition when the cutting tool is secured in the bore and rotated during cutting operations. What potentially results is an improvement in cutting performance because the runout of the assembled tool and holder is reduced, says the company.
For example, a standard 1" toolholder bore diameter is 1.0001," and the end mill for that holder is 0.9997." In addition, the clearance between the diameters is 0.0004." With the 0.0003" eccentric bore, as the set screw is tightened, the clearance is pushed to within 0.0001" of the center, while the standard holder maintains the 0.0004" clearance and the clearances are pushed 0.0004" off-center. When a set screw is turned down into the standard holder and the tool is rotated 180 degrees, the total runout is 0.0008."